By way of background explanation, the cellular radio telephone system divides a geographical area into a plurality of adjoining cells each including a stationary transmitting and receiving base station connected to the conventional fixed-wire telephone network. Each cell is designed to allow a cellular telephone within the cell to access the fixed-wire network, and vice versa. Hence, the particular cell which receives the transmitting and/or receiving signal of a cellular telephone is the cell which is made operative for transmitting and/or receiving the call. So, for example, when a cellular telephone user attempts to make a call, the cell whose base station receives the strongest signal will be the one which effects the connection for that call. If the cellular telephone then moves beyond the effective range of the cell originally handling the call, another cell which receives the signal more strongly will become operative by an automatic transfer or `hand-over` system. Hand-over may occur many times during a call depending on the distance travelled by the telephone and the number of effective cells through which the telephone passes.
Each cellular telephone includes a call-channel selector which controls a frequency synthesizer to span the available call-channel frequencies and, in combination with a microprocessor, selects the call-channel received most strongly, i.e. from the cell base station closest to the telephone.
Cellular radio telephones fall into two main categories, namely mobiles and hand portables, although there is also an intermediate category known as transportables. As the name implies, a hand portable telephone is relatively lightweight and small in size and operates from an internal, rechargeable battery pack so that it may readily be carried around by the user. A mobile telephone, on the other hand, is permanently installed in a vehicle and operates from the vehicle's own (higher voltage) power supply. A mobile has the advantage over a hand portable that it can transmit at a higher power level, but a hand portable has the advantage that the user does not need to be in or near a vehicle to use the telephone, since it can be used anywhere within range of a base station. Unlike a mobile telephone, however, a hand portable does have the drawback of having a limited stand-by and call time before the internal battery pack needs to be recharged. Thus, mobiles and hand portables each have their own particular benefits, and it is not uncommon for a subscriber to have both types of telephone, each with its own unique identity, that is to say the mobile telephone would have one telephone number and the hand portable would have a different telephone number, each with its own billing account. Alternatively, a hand portable telephone may be used in the vehicle with a special booster unit operated from the vehicle's power supply, but this involves additional apparatus and more expense.
Conventionally, each cellular telephone--whether mobile or hand portable--has its own unique telephone number by which it may be accessed from other cellular telephones or fixed-wire conventional telephones. The full telephone number is stored in a memory, e.g. a ROM, PROM or RAM known in the art as a `number assignment module` (NAM) coupled to the control circuits, usually a microprocessor, of the cellular telephone. In the case of mobile telephones the NAM is traditionally contained in the so-called transceiver unit located in a convenient (usually concealed) place separate and remote from the handset, the handset being connected to the transceiver unit.
It is noted that the terms `number assignment module` and `NAM` as used herein encompass any memory means, not necessarily modular in form, nor dedicated to the NAM function, in which information pertaining to the identity of the telephone, specifically the telephone number, is stored.
U.S. Pat. No. 4,677,653 and U.S. Pat. No. 4,734,928 disclose an adapter for a cellular mobile telephone which converts the telephone to have any one of several different telephone numbers selected manually by the user. The adapter is provided with a plurality of number assignment modules (i.e. NAMs) in the form of respective ROMs, each storing a unique telephone number. The ROMs (or NAMs) are mounted on a main circuit board in a housing separate from the telephone unit. Each NAM is connected to a multi-position switch so that a selected one of the NAMs may be coupled to the microprocessor of the telephone to change the telephone number. These U.S. patents essentially extend the concept of a cellular telephone having its own unique telephone number by teaching that each telephone may indeed be adapted to have several unique numbers, any one of which may be active (all the others being dormant) at any one time and the user would have a different billing account for each number. Although the same principle could also be applied to hand portable telephones, the overriding doctrine remains to provide cellular telephones with exclusive telephone numbers, that is to say each telephone either has its own individual telephone number or a unique set of telephone numbers exclusive to that particular telephone.
In the neighbouring art of cordless radio telephones, British Patents Nos. 2,154,395 and 2,159,372 disclose an arrangement of handset and base unit, in which the base unit automatically generates and stores an identification code, and then transmits the same code to the handset over the air. The handset stores the code in memory. When an incoming or outgoing call is initiated, the two identification codes stored in the base unit and the handset, respectively, are compared and only if they match is the call allowed to proceed. The identification codes stored in the base unit and handset are thus provided for security purposes i.e. to prevent crosstalk and the risk of wire tapping in that they prevent a handset being used with any base unit other than its intended partner. Provision is made to enable a plurality of handsets to be operable with a common base unit, but each of the handsets can be used only with the particular base unit (or units) to which it is paired and no other. Moreover, in the case of cordless telephones it is noted that the base unit and the handset, although separate, constitute a single telephone and therefore only have one unique telephone number associated therewith. Furthermore, it is noted that this telephone number does not need to be stored in memory within any part of the telephone since the base unit is connected to the fixed-wire telephone network and hence can be addressed in the conventional manner. There is therefore no equivalent of a number assignment module in the case of a cordless telephone.